Technology is a disruptive force that has toppled business empires. Computers did in typewriters. The telephone ended the short reign of the telegraph.

Now, Magicjack might spell doom for traditional phone companies.

Simply plugging the $39.99 device into the USB port of a computer with a broadband Internet connection installs the software needed to make Voice over Internet Protocol calls with a normal phone.

Users are prompted for a home area code. More than 100 are available, and it is not necessary to select the area code of your region. Within a couple of hours, a phone number is assigned.

Plug a phone into the other end of Magicjack and that's it . . . you have phone service.

Call quality is excellent, comparable to a land line and noticeably better than a cellphone.

For those who have abandoned their land line and use cellphones exclusively, Magicjack provides a cheap alternative —- perfect for saving cellphone minutes or for calling the baby sitter back at home.

Pros

> Low cost. Yearly service is $19.99, less than $2 a month. The first year of service is included in the initial $39.99 purchase price. Voice mail, caller ID, call waiting and call forwarding are included.

> Phone service travels anywhere you take a laptop and have broadband Internet service, including overseas.

> International rates are low. A phone call to a land line in England costs about 2 cents a minute, for example. Phone calls between Magicjack numbers are always free, even when one or both callers are overseas.

> 911 emergency phone calls work with Magicjack.

> The Magicjack number is not listed in any phone directory.

Cons

> The computer must remain on to make or receive phone calls.

> Does not work with dial-up Internet service. Phone calls require a minimum 80 kb/s Internet connection.

> One Magicjack provides one phone. Larger homes should consider a multi-handset cordless phone.

> Only works with PCs and Intel-based Macs. A Linux-compatible version is in the works.

Tips

> Some wireless routers interfere with cordless phones. For best results, avoid 2.4 and 5.8 ghz handsets. Older 900 mhz phones work well, as do newer DECT phones. Or simply use a corded phone.

> If shopping for a cordless phone, don't bother getting one with voice mail or an answering machine. These services are included with Magicjack. Thrift stores frequently have suitable cordless phones for less than $5.

> An old laptop nestled in a corner is enough to keep Magicjack online, though sleep and hibernation modes can interfere with service. In testing, a laptop as slow as 500 mhz with a wireless Internet connection proved functional.